Medical knife

ABSTRACT

Provided is a medical knife which can be more easily handled and makes it possible to maintain low penetration resistance, without the maximum-width parts of said medical knife creating sudden changes in penetration resistance even when passing through tissue. A knife (A) comprises: a pointed tip ( 4 ); a cutting part ( 1 ) which has edges ( 5 ) consisting of first oblique faces ( 6 ) which are formed along the outer periphery and second oblique faces ( 8 ) which are formed inside the first oblique faces ( 6 ); maximum-width parts ( 2 ) which are formed contiguously with the cutting part ( 1 ); and a shank part ( 3 ) which is formed contiguously with the maximum-width parts ( 2 ). The cutting part ( 1 ) is formed in such a way that the shape of the planar projection thereof becomes wider from the pointed tip ( 4 ) towards the maximum-width parts ( 2 ), and the first oblique faces ( 6 ) extend further towards the shank part ( 3 ) side than the second oblique faces ( 8 ) over at least one portion of the area running from the maximum-width parts ( 2 ) to the shank part ( 3 ).

TECHNICAL FIELD

The present invention relates to a medical knife capable of making anincision with small penetration resistance, which does not generatedrastic change in the penetration resistance even when a maximum-widthpart passes, when incising an affected area.

BACKGROUND ART

In a cataract operation, a cornea and a sclera are incised with a widthcorresponding to a dimension of a lens to be embedded for embedding thelens in an eyeball. As the medical knife used when incising the eyeball,there is one of which planar shape is formed into a substantiallyrhombic shape. Such medical knife has a sharp tip, a cutting part havingtwo edges radially formed from the tip, a maximum-width part in which awidth dimension of the cutting part is the largest, and a shank partcontinuously formed from the maximum-width part of which width dimensiondecreases as is away from the maximum-width part (for example, refer toPatent Document 1).

In a case of the above-described medical knife, by allowing the same tostraightly penetrate the eyeball, an incision corresponding to thedimension of the maximum-width part may be formed. Especially, in themedical knife disclosed in Patent Document 1, durability of the sharptip may be improved, and since the edges are formed on both sides fromthe maximum-width part to the shank part, the penetration resistance maybe made small.

In the medical knife disclosed in Patent Document 1, the edges areformed on the both sides in the maximum-width part and the edges areformed also from the maximum-width part to the shank part. Therefore,high incision performance to the tissue is maintained of course from thetip to the maximum-width part and even after the maximum-width partpasses through the tissue.

Therefore, the penetration resistance generated when incising theeyeball with the above-described medical knife is a substantiallysimilar small value from the tip to the maximum-width part and thesubstantially similar penetration resistance is maintained even afterthe maximum-width part passes through the tissue. When the maximum-widthpart passes through the tissue and the width of the knife decreasestoward the shank, the penetration resistance drastically decreases.

-   Patent Document 1: Japanese Patent No. 4161026

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

When incising the eyeball with the medical knife, a part with thelargest resistance is the cornea and the sclera, and the resistance in acrystal lens is small. That is to say, when performing incisionoperation by allowing the sharp tip of the medical knife to face thetissue, the penetration resistance drastically increases to achieve thelargest value at a stage in which the tip penetrates the tissue to formthe initial incision. Thereafter, the penetration resistance isgenerated also when the edge continuous from the sharp tip expands thetissue. Then, after the maximum-width part passes through the tissue,the penetration resistance decreases. In this manner, when making anincision with the medical knife, the penetration resistance changes inaccordance with progress in incision.

The change in the penetration resistance is preferably small. Forexample, when the change between the penetration resistance when formingthe initial incision to the tissue and the penetration resistance whenexpanding the initial incision is drastic and large, since a doctorhandles the medical knife by applying force necessary to form theinitial incision, excessive force acts on the tissue from a moment thatthe initial incision is formed and a penetration speed might increase tocause excessive incision. Also, when the maximum-width part of themedical knife passes through the tissue, when the change between thepenetration resistance when expanding the initial incision and thepenetration resistance after the maximum-width part passes through thetissue is drastic and large, since the doctor handles the medical knifeby applying force necessary to expand the tissue, the excessive forceacts on the tissue from a moment that the maximum-width part passesthrough the tissue and the penetration speed might increase to cause theexcessive incision.

Therefore, it is required that the doctor who handles the medical knifesensitively responds to slight change in the penetration resistancesensed by hand and finger, so that delicate handling and sensitive senseare required for recognizing the change in the penetration resistance,and there is a problem of fatigue in an operation.

An object of the present invention is to provide the medical knifecapable of maintaining the small penetration resistance, not generatingthe drastic change in the penetration resistance even when themaximum-width part passes through the tissue, and improvinghandleability.

Means for Solving Problem

In order to solve the above-described problem, the medical knifeaccording to the present invention includes a sharp tip, a cutting partincluding an edge formed of a first oblique face formed along anouter'periphery and a second oblique face formed inside the firstoblique face, a maximum-width part formed continuously from the cuttingpart, and a shank part formed continuously from the maximum-width part,wherein a planar projection shape of the cutting part is formed suchthat a width dimension increases from a side of the sharp tip toward themaximum-width part, and the first oblique face extends further toward aside of the shank part than the second oblique face on at least a partof a portion from the maximum-width part to the shank part side.

Effect of the Invention

In the above-described medical knife (hereinafter, simply referred to asthe “knife”), the penetration resistance generated when incising thetissue may be maintained small and the penetration resistance does notdrastically change even when the maximum-width part passes through thetissue. Therefore, the doctor may make weight on sensitivity toapplication of the force small when handling to reduce the fatigue.

That is to say, in the knife of the present invention, in the planarshape (planar projection shape), the width dimension increases from thesharp tip to the maximum-width part and the shank part is formedcontinuously from the maximum-width part. Also, the cutting part has theedge formed of the first oblique face and the second oblique face formedinside the first oblique face. The first oblique face extends furthertoward the shank part side than the second oblique face on at least apart of the portion from the maximum-width part to the shank part.

In the knife of the present invention configured as above, in the planarshape in the maximum-width part, the first oblique face to form the edgeobliquely intersects with the maximum-width part of the knife and thefirst oblique face is formed along an outer periphery of themaximum-width part to the shank part side. Therefore, a substantiallytriangle wedge-shaped surface having an intersection with the edge as anapex and is defined by an end edge of the first oblique face and an endedge of a lower oblique face of which area increases from the edge sidetoward the shank part side is formed on a side surface of the knife.

Also, since the first oblique face is formed so as to be overlapped withthe second oblique face on at least apart of the portion from themaximum-width part to the shank part side, when it is configured suchthat the width dimension gradually decreases from the maximum-width parttoward the shank part, a cross-sectional area from the maximum-widthpart on which the first and second oblique faces are formed to the shankpart side may be made substantially constant.

Therefore, the knife of the present invention may maintain thepenetration resistance small because the edge is formed on the portionfrom the sharp tip to the maximum-width part, and may maintain the smallpenetration resistance after forming the initial incision to the tissueto the maximum-width part because the edges are formed on the both sidesof the maximum-width part.

After the maximum-width part of the knife passes through the tissue byfurther handling of the knife, the edges are not formed on the bothsides of the knife, so that the penetration resistance larger than thepenetration resistance generated when the edge passes through the tissueis generated. At that time, since a shape of the side surface of themaximum-width part of the knife is the substantially wedge-shape, thisacts in a direction in which the penetration resistance increasestogether with increase in the penetration, on the other hand, the firstoblique face is formed so as to be overlapped with the second obliqueface at least a part of the portion from the maximum-width part to theshank part side and the width dimension decreases, and this acts in thedirection in which the penetration resistance decreases, so that thegenerated penetration resistance becomes substantially constant in thevicinity of the maximum-width part. At the same time, after themaximum-width part passes through the tissue, the width of the knifedecreases, so that this decreases together with this.

Therefore, the penetration resistance is continuously generated alsoafter the maximum-width part of the knife passes through the tissue andthe doctor is not affected by the drastic change in the penetrationresistance. Therefore, the doctor may recognize the change in thepenetration resistance while handling the knife, thereby recognizingthat the incision of the tissue is completed. Also, since the drasticchange in the penetration resistance is not generated, there is a littlepossibility of forming the excessive incision due to too much forceapplied when the maximum-width part passes and it is possible to makethe weight on the sensitivity to the application of the force small.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an orthographic view of a knife according to this embodiment.

FIG. 2 is a perspective view of the knife.

FIG. 3 is plan views of knives of comparative examples A to D, F and Gand the knife of the present invention E.

FIG. 4 is views illustrating measured values of penetration resistanceby the knives of the comparative examples A to D, F and G and the knifeof the present invention E.

DESCRIPTION OF LETTERS OR NUMERALS

-   -   A knife    -   1 cutting part    -   2 maximum-width part    -   3 shank part    -   4 tip    -   5 edge    -   6 first oblique face    -   7 lower oblique face    -   8 second oblique face    -   9 upper flat surface    -   6 a, 8 a boundary line    -   10 lower flat surface    -   11 side surface

BEST MODE(S) FOR CARRYING OUT THE INVENTION

Hereinafter, a preferred embodiment of a medical knife according to thepresent invention is described with reference to the drawings. FIG. 1 isan orthographic view of the knife according to this embodiment. FIG. 2is a perspective view of the knife.

The knife according to the present invention has a function to penetratea tissue of an eyeball and the like to form an initial incision and makean incision in a width direction from the initial incision as a startingpoint to expand the same in a thickness direction. Especially, it isconfigured such that drastic difference is not generated betweenpenetration resistance generated when the knife penetrates the tissue toincise and the penetration resistance just after a maximum-width part ofthe knife passes through the tissue.

A material to form the knife according to the present invention is notespecially limited, and steels such as stainless steel and carbon steelmay be used. However, in view of an antirust property and easierworking, the stainless steel is preferably used and among others,austenitic stainless steel is desirably used.

When the austenitic stainless steel is used as the material, it ispreferable to apply cold plastic working at a predetermined working rateto extend a structure in a fiber shape and apply press working, polishworking and another necessary working while maintaining the fiber-shapedstructure in order to increase hardness of the material.

In FIGS. 1 and 2, a knife A is composed of a cutting part 1, amaximum-width part 2 on a widest position of the cutting part 1, and ashank part 3 formed continuously from the maximum-width part 2. A tip ofthe cutting part 1 is formed as a sharp tip 4 and edges 5 are formed onboth sides from the tip 4 as the starting point toward the maximum-widthparts 2. Also, it is formed such that a width dimension graduallydecreases from the maximum-width part 2 toward the shank part 3.Therefore, a planar shape (planar projection shape) from the cuttingpart 1 through the maximum-width part 2 to the shank part 3 of the knifeA is formed into a substantially rhombic shape.

Detailed dimensions such as a length dimension from the tip 4 to themaximum-width part 2 and the width dimension of the maximum-width part 2are variously set according to an object of the knife A, for example, aspecification such as a dimension of the incision.

Also, the planar shape of the maximum-width part 2 is not especiallylimited, the knife A may have a rhombic planar shape with themaximum-width parts 2 as apices and the maximum-width part 2 may beslightly linearly formed in a longitudinal direction of the knife A.

The edge 5 is formed along an outer periphery of the cutting part 1. Theedge 5 has a function to incise the tissue including a cornea and asclera and is formed of first oblique face 6 formed on an upper side ofa plane including two edges 5 formed on the both sides from the tip 4 tothe maximum-width part 2 as a boundary and a lower oblique face 7 formedon a lower side thereof intersecting with each other. In this manner,since the first oblique face 6 and the lower oblique face 7 form theedge 5 together, the oblique faces 6 and 7 are formed as ground surfaceswith high smoothness.

A second oblique face 8 is formed inside the first oblique face 6 alongthe first oblique face 6 formed on the upper side of the edge 5 of thecutting part 1, and a boundary line 6 a is formed on a boundary betweenthe both oblique faces 6 and 8. Since the second oblique face 8 does nothave a function to form the edge 5, it is not necessarily required toform the same of the ground surface with the high smoothness and thismay be a pressed surface or a roughly ground surface. Also, an upperflat surface 9 is formed along the second oblique face 8 and the upperflat surface 9 also may be the pressed surface or the roughly groundsurface as the second oblique face 8.

A lower flat surface 10 is formed continuously from the lower obliqueface 7 formed on the lower side of the edge 5 of the cutting part 1.Since the lower flat surface 10 is formed as a surface, which does nothave the function to form the edge 5, it is not necessarily required toform the same of the ground surface with the high smoothness and thismay be formed as the roughly ground surface or the pressed surface.

In the cutting part 1, the first oblique face 6 is formed of a flatsurface and this is formed to have the same width (dimension from theedge 5 to the boundary line 6 a) from the tip 4 to the vicinity of themaximum-width part 2. The lower oblique face 7 formed on the lower sideof the plane including the two edges 5 also is formed as the flatsurface. Therefore, the edge 5 has a pointed sharp shape by intersectionof the two flat surfaces. In such cutting part 1, the boundary line 6 abeing the boundary between the first oblique face 6 and the secondoblique face 8 is formed as a well-defined ridge.

The above-described first oblique face 6 and lower oblique face 7 may beformed by using a grinding material having high flatness and moderaterigidity. As such grinding material, there are a grindstone, abelt-shaped grindstone of which rear surface is supported by a highlyrigid frame and the like, and they may be selectively used.

The second oblique face 8 is formed inside the first oblique face 6along the first oblique face 6. The second oblique face 8 is formed suchthat an angle relative to the plane including the two edges 5 is smallerthan an angle of the first oblique face 6. The second oblique face 8 issubstantially linearly formed along the first oblique face 6 through themaximum-width part 2 to a side surface to reach the shank part 3. Thus,a boundary line 8 a being a boundary between the second oblique face 8and the upper flat surface 9 is formed as a well-defined linear ridge.

The two edges 5 are formed from the tip 4 as the starting point, thefirst oblique faces 6 are formed along the edges 5, and further thesecond oblique faces 8 are formed inside the first oblique faces 6 alongthe same, so that a line 6 b on which the two first oblique faces 6intersect with each other and a line 8 b on which the two second obliquefaces 8 intersect with each other become continuous straight linesbeginning from the tip 4.

The first oblique face 6 extends further toward a shank part 3 side thanthe second oblique face at least a part of a portion from themaximum-width part 2 to the shank part 3 side. In this embodiment, thefirst oblique face 6 is formed on an outer peripheral side of the secondoblique face 8 on a portion from the cutting part 1 through themaximum-width part 2 to the shank 3 of the knife A.

Therefore, the first oblique face 6 extends further toward the shankpart 3 side than the second oblique face 8 on the portion from themaximum-width part 2 to the shank part 3. However, it is not requiredthat the first oblique face 6 is formed over an entire length of thesecond oblique face 8 formed from the maximum-width part 2 to the shankpart 3 and this may be formed on a part thereof.

As described above, the knife A is such that the first oblique face 6,the second oblique face 8, and the upper flat surface 9 are formed onthe upper side of the plane including the two edges 5 of the cuttingpart 1, and the lower oblique face 7 and the lower flat surface 10 areformed on the lower side thereof. Therefore, as a shape in a side viewof the knife A, the edge 5, the ridge 6 a, the upper flat surface 9, andthe lower flat surface 10 are represented in a part corresponding to thecutting part 1.

Also, in a side view of the vicinity of the maximum-width part 2 of theknife A, a point on which the edge 5 intersects with the maximum-widthpart 2 is made an apex, the first oblique face 6 and the lower obliqueface 7 form two sides away from each other toward the shank part 3 side,and a triangle-shaped side surface 11 by a line on which the firstoblique face 6 intersects with the upper flat surface 9 and a line onwhich the lower oblique face 7 intersects with the lower flat surface 10is formed.

On the side surface 11, a corner on the lower side forms an obtuse ridgeformed by intersection of the lower oblique face 7, and a corner on theupper side forms an obtuse ridge 11 a formed by intersection of thefirst oblique face 6. Especially, an angle formed by the ridge 11 a onthe upper side of the side surface 11 (angle between the first obliqueface 6 and the side surface 11) becomes larger than the angle formedwhen the second oblique face 8 directly intersects with the side surface11, and it is possible to significantly decrease the function to incisethe tissue. That is to say, the first oblique face 6 further chamfers acorner portion on which the side surface 11 and the second oblique face8 intersect with each other, and the ridge 11 a on the upper side of theside surface 11 is formed with the large angle.

In this manner, the first oblique face 6 further extending furthertoward the shank part 3 side than the second oblique face 8 from themaximum-width part 2 to the shank part 3 does not intersect with thelower oblique face 7 but intersects with the side surface 11. Therefore,the edge is not formed on an outer peripheral portion of the firstoblique face 6 extending from the maximum-width part 2 to the shank part3 side.

The knife A composed as above has an excellent incision performance tothe tissue by the sharp tip 4 and the edge 5 in the cutting part 1 fromthe tip 4 to the maximum-width part 2 and is able to decrease thepenetration resistance when incising the tissue. Also, the maximum-widthpart 2 has the incision performance only on an apex portion on which theedge intersects with the side surface 11. However, the edge 5 disappearsat the same time as passing through the maximum-width part 2, andfurther a thickness of the both end portions in the width direction ofthe knife A increases together with the triangle on the side surface 11,so that resistance (frictional resistance) to expand an incision site isgenerated when passing through the tissue. Further, since the widthdimension of the knife A decreases at the same time as passing throughthe maximum-width part 2, the side surface 11 is separated from thetissue and the resistance when passing through the tissue decreases.

Therefore, until the maximum-width part 2 of the knife A passes throughthe tissue, the resistance when the edge 5 incises the tissue isgenerated as the penetration resistance, and soon after themaximum-width part 2 passes through the tissue, difference between theresistance generated when expanding the tissue and decrease in theresistance by separation of the side surface 11 from the tissue isgenerated on the maximum-width part as the penetration resistance. Thatis to say, after the maximum-width part 2 of the knife A passes throughthe tissue, the penetration resistance does not drastically decrease butgradually changes. Therefore, a doctor may easily recognize that themaximum-width part 2 passes through the tissue and may surely handlewith modulation.

Next, a process to manufacture the knife A in which the first obliqueface 6 extends further toward the shank part 3 side than the secondoblique face 2 in at least a part of the portion from the maximum-widthpart 2 to the shank 3 side as described above is simply described.

First, a material corresponding to the planar shape of a destinationknife A is formed. In this case, when the material is the steel ormartensite stainless steel, it is sufficient to cut out a flat platewith the thickness of the knife A into the rhombic shape by pressing.Also, when the material is the austenitic stainless steel and the like,the press working may be applied to a line material having the structureto which cold withdrawing working is applied in advance to be extendedin the fiber shape to form the same into a rhombic flat plate shape.

The second oblique face 8 is formed on the rhombic material formed inthe above-described manner by pressing or grinding, and thereafter, byforming the first oblique face 6 on a portion from the tip 4 through themaximum-width part 2 to the shank part 3 side by grinding the outerperipheral side of the second oblique face 8, it is possible to extendthe first oblique face 6 further toward the shank part 3 side than thesecond oblique face on at least a part of the portion from themaximum-width part 2 to the shank part 3 side.

Then, the knife A may be manufactured through another process includinga predetermined polishing process after forming the lower oblique face7.

When forming the first oblique face 6 on the rhombic material, bygrinding along the edge 5 and thereafter grinding along the side surface11 from the maximum-width part 2 to the shank part 3 side by changing anattitude of the material, it is possible to extend the first obliqueface further toward the shank part 3 side than the second oblique faceon a part of the portion from the maximum-width part 2 to the shank part3 side.

However, it is preferable to continuously perform the grinding along theedge 5 and the grinding along the side surface 11 without performingsuch grinding in two steps. In this case, the attitude of the materialmay be continuously changed according to the grinding corresponding tothe edge 5 and the grinding corresponding to the side surface 11. Whenperforming such grinding, in the vicinity of the maximum-width part 2 ofthe knife A, the boundary line 6 a between the first oblique face 6 andthe second oblique face 8 becomes a curved-line shape and the width ofthe first oblique face 6 (dimension from the edge 5 to the boundary line6 a and dimension from the side surface 11 to the boundary line 6 a)increases.

It is also possible to form the first oblique face 6 in a precedentprocess and form the second oblique face 8 in a successive process.

Next, a result of comparing the penetration resistance of the knife Aaccording to the present invention and the penetration resistance of aknife according to a comparative example is described. FIG. 3 is planviews of knives of comparative examples A to D, F and G and the knife ofthe present invention E. FIG. 4 is views illustrating measured values ofthe penetration resistance by the knives of the comparative examples Ato D, F and G and the knife of the present invention E.

Meanwhile, an experiment to measure the penetration resistance wasperformed by allowing the knife to be tested to vertically penetrate thesclera of a swine eye cut to have a thickness of 400 μm to 450 μm as amaterial to be penetrated and measuring the resistance by a load cell.The penetration was performed three times for each of three tests.

In FIGS. 3 and 4, in the knife of the comparative example A, the edge ina gradual curved-line shape is formed from the sharp tip through themaximum-width part to the shank part side.

In the knife of the comparative example A, the penetration resistancewhen forming the initial incision to the material to be penetrated iswithin a range from approximately 160 mN (millinewton, the same shallapply hereinafter) to approximately 95 mN. Also, the penetrationresistance relatively monotonously decreases after forming the initialincision and drastically decreases just before a lapse of 2 seconds(time at which the maximum-width part passes through the material to bepenetrated). In this case, since the penetration resistance is small, itmay be said that this has the excellent incision performance.

In the knife of the comparative example A, the tip is sharp and the edgeis formed through the maximum-width part to the shank part side, so thatthe penetration resistance is small and this has the excellent incisionperformance. However, variation in measured values is large, thepenetration resistance monotonously decreases, and a change rate islarge. Therefore, it might be difficult for the doctor to clearlyrecognize a time point at which the maximum-width part passes throughthe tissue.

In the knife of the comparative example B, a substantially linear edgeis formed from the sharp tip to the maximum-width part, the edge is notformed from the maximum-width part to the shank part side, and the widthdimension linearly decreases.

In the knife of the comparative example B, the penetration resistancewhen forming the initial incision to the material to be penetrated iswithin a range from approximately 300 mN to approximately 190 mN. Thepenetration resistance drastically decreases after forming the initialincision to be maintained at a substantially constant value anddrastically decreases just before the lapse of 2 seconds. Especially,the substantially constant penetration resistance when penetrating thematerial to be penetrated is within a range from approximately 220 mN toapproximately 140 mN.

In the knife of the comparative example B, the penetration resistance islarge, and the incision performance thereof is poor. Also, the change inthe penetration resistance just before the lapse of 2 seconds is largeand it might be difficult for the doctor to recognize that themaximum-width part of the knife passes through the tissue when incisingthe same.

In the knife of the comparative example C, the edge in the gradualcurved-line shape is formed from the sharp tip to the maximum-widthpart, the edge is not formed from the maximum-width part to the shankpart and the width linearly decreases.

In the knife of the comparative example C, the penetration resistancewhen forming the initial incision to the material to be penetrated iswithin a range from approximately 210 mN to approximately 110 mN. Thechange in the penetration resistance gradually decreases after formingthe initial incision and relatively drastically decreases just beforethe lapse of 2 seconds.

In the knife of the comparative example C, the penetration resistance isnot especially large but is not a small value. Also, the change justbefore the lapse of 2 seconds is large. Therefore, it might be difficultfor the doctor to recognize that the maximum-width part of the knifepasses.

In the knife of the comparative example D, the edge is substantiallylinearly formed from the sharp tip to the maximum-width part, the widthgradually decreases from the maximum-width part to the shank part, andthereafter, the width decreases in a curved-line shape convex inward.

In the knife of the comparative example D, the penetration resistancewhen forming the initial incision to the material to be penetrated iswithin a range from approximately 180 mN to approximately 120 mN. Thechange in the penetration resistance is small and after forming theinitial incision to the tissue, the penetration resistance issubstantially constant and drastically decreases just before the lapseof 2 seconds.

In the knife of the comparative example D, the penetration resistance isnot especially large but is not a small value. Especially, since thepenetration resistance after forming the initial incision to thematerial to be penetrated is substantially constant, the change justbefore the lapse of 2 seconds is extremely large. Therefore, it might bedifficult for the doctor to recognize that the maximum-width part of theknife passes.

The knife A according to the present invention is illustrated in E. Inthis knife A, the penetration resistance when forming the initialincision to the material to be penetrated is within a range fromapproximately 100 mN to approximately 60 mN. Also, after forming theinitial incision, the penetration resistance slightly decreases but thechange is not large. This relatively gradually decreases just before thelapse of 2 seconds.

In the knife A according to the present invention (E), the penetrationresistance is small, the change after forming the initial incision issmall, and further, the change just before the lapse of 2 seconds isgradual. Therefore, the doctor may decrease the force when allowing theknife to pass through the tissue, and it is also possible to make anincision by applying constant force. Further, the change in thepenetration resistance when the maximum-width part passes through thetissue also is gradual and the doctor may easily recognize that themaximum-width part of the knife passes through the tissue.

The knife of the comparative example F has a shape substantiallyidentical to that of the knife of the comparative example D. That is tosay, the knife of the comparative example F is such that the edge issubstantially linearly formed from the sharp tip to the maximum-widthpart, the width gradually decreases from the maximum-width part to theshank part, and thereafter, the width decreases in the curved-line shapeconvex inward.

In the knife of the comparative example F, the penetration resistancewhen forming the initial incision to the material to be penetrated iswithin a range from approximately 160 mN to approximately 130 mN. Thechange in the penetration resistance is small, the penetrationresistance further increases after forming the initial incision to thetissue and drastically decreases just before the lapse of 2 seconds.

In the knife of the comparative example F, the penetration resistance isnot especially large but is not a small value. Especially, after formingthe initial incision to the material to be penetrated, the penetrationresistance increases and the change just before the lapse of 2 secondsis extremely large. Therefore, it might be difficult for the doctor torecognize that the maximum-width part of the knife passes.

In the knife of the comparative example G, the edge is substantiallylinearly formed from the sharp tip to the maximum-width part, a lengthof the maximum-width part is substantially identical to the length ofthe cutting part, the width slightly decreases at the part toward theshank part, and thereafter, the width decreases in the curved-line shapeconvex inward.

In the knife of the comparative example G, the penetration resistancewhen forming the initial incision to the material to be penetrated iswithin a range from approximately 270 mN to approximately 170 mN. Thepenetration resistance further increases after forming the initialincision to the tissue and drastically decreases just before the lapseof 2 seconds.

In the knife of the comparative example G, the penetration resistance isespecially large. Also, after forming the initial incision to thematerial to be penetrated, the penetration resistance increases and thechange just before the lapse of 2 seconds is extremely large. Therefore,a load when the doctor handles is large and it might be difficult torecognize that the maximum-width part of the knife passes.

As in the above-described experimental result, in the knife A of thepresent invention, a value of the largest penetration resistancegenerated when forming the initial incision to the material to bepenetrated is 100 mN and is the smallest as compared to that of othercomparative examples. Therefore, the knife A of the present invention ismost excellent in the penetration performance.

They are the knives of the comparative examples A and C and the knife Aof the present invention in which the change of the penetrationresistance just before the lapse of 2 seconds is small (inclination of achange curve is gradual), and among others, the change in the knife A ofthe present invention is the smallest. Therefore, in the knife A of thepresent invention, when the doctor handles the same by applyingsubstantially identical force, even when the maximum-width part of theknife A passes through the tissue, excessive force is not applied to thetissue and there is no possibility of forming an excessive incision.

As described above, in the knife A of the present invention, thepenetration resistance when forming the initial incision is small, thechange in the penetration resistance after forming the initial incisionuntil the maximum-width part passes therethrough is small, and further,the penetration resistance is present also after the maximum-width partpasses. Therefore, the doctor may allow the knife to pass through thetissue by applying small force and may incise the tissue bysubstantially identical force. Then, since the change in the penetrationresistance is small also before and after the maximum-width part passesthrough the tissue, there is a little possibility of incising the tissueexcessively also when the doctor handles by applying the substantiallyidentical force.

INDUSTRIAL APPLICABILITY

The knife according to the present invention has no possibility offorming the excessive incision to the tissue when penetrating the tissueto incise. Therefore, this may be utilized not only to an operation oneye but also to an operation to make an incision while maintaining thedimension of the incision constant.

1. A medical knife, comprising: a sharp tip; a cutting part including afirst oblique face formed along an outer periphery, an edge formed ofthe first oblique face, and a second oblique face formed inside thefirst oblique face; a maximum-width part formed continuously from thecutting part; and a shank part formed continuously from themaximum-width part, wherein a planar projection shape of the cuttingpart is formed such that a width dimension increases from a side of thesharp tip toward the maximum-width part, and the first oblique faceextends further toward a side of the shank part than the second obliqueface on at least a part of a portion from the maximum-width part to theshank part side.
 2. The medical knife according to claim 1, wherein thecutting part includes a lower oblique face formed on a lower side of thefirst oblique face, and the edge is formed of the first oblique faceformed on an upper side and the lower oblique face formed on the lowerside intersecting with each other.
 3. The medical knife according toclaim 2, wherein the first oblique face and the lower oblique face areformed as ground surfaces of which smoothness is higher than thesmoothness of the second oblique face.